Internal fixation is indicated for many orthopedic conditions resulting from disease, injury or deformity. Spinal implant systems can be used to temporarily stabilize the spine and maintain certain vertebrae in rigid position relative to each other while awaiting solid fusions of segments of the spine. Most spinal implant systems include bone plates and spinal rods which are fixed to the spine with various devices, such as bone screws, bolts, wires and connectors.
Various orthopedic techniques and systems have been developed to engage a bone plate or other orthopedic implant securely to bone. Implants are sometimes press fitted between two cortical walls or attached to the bone with bone screws, including cancellous, cortical and bicortical screws, or various wires and cables, hooks, clamps and pins. Unfortunately, complications can develop with many of these fixation methods. For example, bone screws can loosen over time, pull out or thread themselves back out. The associated failure of the bone to provide adequate support for the screw threads is heightened in weaker bone, such as found in older rheumatoid and osteoporitic patients. Bone screw failures often require revision and replacement with a larger replacement screw. This is particularly problematic because often a larger screw cannot be used.
Bone screws have a further disadvantage for spinal applications in that they usually require bicortical placement for secure fixation. The tip of the screw which extends out of the opposite side of the bone can damage soft tissue structures leading to obvious complications. When the site of fixation is the vertebral column, bicortical placement of the screws risks spinal cord injury which can lead to paralysis.
Several devices have been developed in an attempt to answer the limitations of the bone screws. For example, surgical toggle bolt or molly bolt approaches have been used as disclosed in U.S. Pat. No. 2,485,531 to Dzus, U.S. Pat. No. 5,098,433 to Freedland and German Patent No. 3509-417A. In Freedland and Dzus, a pivoting member rotates into a position to anchor the fastener. In the German reference, the arms of the anchoring device are separated and pressed into contact as a screw is threaded through the center of the device. In spite of these efforts, toggle bolts have not presented a viable answer because installation is time consuming and requires the application of torque to the bone. Furthermore, toggle bolt removal is extremely difficult and can cause extensive damage to the bone.
U.S. Pat. No. 4,716,893 to Fisher discloses a bone anchor for fastening a screw into cancellous bone. An anchor sleeve is fitted into a bore formed in a bone. The anchor sleeve is expanded by the introduction of a threaded screw. The bore must have substantial depth in order to receive the anchor sleeve which may unfortunately place the device in close proximity to vital soft tissue structures. A similar approach is described in U.S. Pat. No. 5,209,753 to Biedermann et al. Biedermann discloses a bone screw which defines a longitudinal bore and a pair of slits. A shaft is screwed into the bore and engaged into a threaded bore in the movable tip of the screw. The tip includes an expander portion. After the screw is engaged to bone, the shaft is unscrewed toward the head of the screw pulling the expander portion into the slitted bore which in turn expands the threaded portion of the screw. One disadvantage of this approach is that a separate removable tool is required to engage the removable tip. Moreover, this device provides no means for controlling the expansion of the slitted bone screw within the bone.
Vives (U.S. Pat. No. 4,711,234) discloses a retention pin assembly for fixing a bone plate or other prosthesis on bone tissue. The assembly is composed of two half pins that are pressed into a bore. The pins are shaped so that they will spread progressively apart as the surface of the opposing pins slidingly engage. The resilient bending of the half pins forces them to press against the wall of a bore formed in the cortical region of the bone.
U.S. Pat. No. 5,167,665 to McKinney teaches attaching orthopedic implants to bone with blind rivets. The McKinney invention is advantageous in that bicortical placement of the fastener is not required. Also, the device does not require applying torque to the bone as is necessary for toggle bolts. However, this device does require the application of force to the cortex of bone to engage the rivets to the bone. This is accomplished with a rivet gun or other tool after a rivet has been placed in a drilled hole. The blind rivets are similar to the toggle bolt approach because the drilled hole must be enlarged if the rivets must be removed and replaced. The enlarged hole may then require a fastener which is too large to used in the particular location.
A need has remained for orthopedic fixation devices which provide rigid fixation of implants to bone, do not require bicortical placement and minimize the intrusion into the cancellous material of the bone during implantation. A need has also remained for improved systems which reduce the trauma to the bone to be instrumented and the surrounding tissues.